The present invention relates to display apparatuses, particularly to display apparatuses such as a transmissive display apparatus capable of displaying images with the optimal contrast condition by optimizing an optical system of an observation environment (i.e., ambient light, observation distance, and display brightness (or brightness of background ambient light and a reflected image), or brightness of a translucent image), thereby improving the visibility.
In recent years, transmissive display apparatuses have been proposed such as a mirror having an image display function or a mirror with a built-in monitor for displaying images taken by a camera and visual information on a rearview mirror, a side mirror or the like, and an information display apparatus for vehicles or a head-up display for projecting and displaying visual information on a windshield, a meter portion of a dashboard, a car navigation display apparatus or the like, and a part of such transmissive apparatuses has been already commercially used. The application of the technique of such transmissive display apparatuses not only to the vehicle technology field but also to fields of aircraft, military, medicine and computer game and other fields has been also proposed.
In relation to such transmissive display apparatuses, the visibility of a displayed image or displayed visual information deteriorates due to the lightness or brightness of a background or environment, or the visibility of a displayed image or displayed visual information superimposed on a landscape image deteriorates due to the changing landscape image. To cope with it, the technique of measuring or estimating the lightness or brightness of a background or environment and controlling the display brightness of a displayed image or displayed visual information in accordance with the lightness or brightness of the background or environment has been proposed. For transmissive display apparatuses, in order to further improve the visibility, controlling the display brightness of a displayed image or displayed visual information in accordance with not only the lightness or brightness of a background or environment but also the fineness thereof has been proposed (for instance, see JP 2013-174667 A).
JP 2013-174667 A discloses a display apparatus for vehicles comprising background brightness estimating means for estimating the brightness at a plurality of points in a front landscape to estimate a background brightness distribution, background spatial frequency estimating means for estimating spatial frequencies in a plurality of regions of the front landscape to estimate a background spatial frequency distribution, and control means for controlling the display brightness based on an estimation value of the background brightness distribution and an estimation value of the background spatial frequency distribution. The display apparatus for vehicles is defined to be able to control the display brightness in accordance with, in addition to the lightness (brightness) of a background at the place where the display is superimposed, the fineness (spatial frequency) of the background owing to the above configuration.